TECHNICAL FIELD
[0001] The present invention relates to a carboxymethylated microfibrillated cellulose fiber
and a composition comprising said fiber.
BACKGROUND ART
[0002] During a papermaking process, a composition prepared by dispersing a pulp and a pigment
in water is used. The water retention ability of such a composition is important from
the viewpoints of increased efficiency of production process and improvement of product
quality. For example, when a base paper is made using a pulp slurry as a raw material,
the water retention ability of the pulp slurry has a great impact on the water drainage
of the slurry on a wire screen and the dispersibility of the pulp, and as a consequence
on the paper strength, air resistance and bulkiness of a produced paper. Further,
the degree of penetration of a binder into a base paper varies depending on the water
retention ability of a pigment coating liquid, and thus, the water retention ability
of a pigment coating liquid has a great impact on the strength and adhesiveness of
a pigment coated layer and a base paper. In recent years, many studies have been actively
conducted on cellulose nanofibers made using cellulose as a raw material. For example,
PTL 1 discloses a technique related to a composition comprising a cellulose nanofiber.
CITATION LIST
PATENT LITERATURE
[0003] PTL 1: Japanese Unexamined Patent Application Publication No.
JP 2017-110085
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0004] The present inventors conceived an idea that if the water retention ability of a
composition can be enhanced by using a microfibrillated cellulose fiber with a lower
degree of defibration than cellulose nanofibers, papers with increased strength can
be produced with low cost. However, no study of such an idea has been conducted yet.
In light of these circumstances, an object of the present invention is to provide
a microfibrillated cellulose fiber that provides a composition having an excellent
water retention ability and exerts an effect to enhance paper strength when added
to a paper.
SOLUTION TO PROBLEM
[0005] The present inventors found that carboxymethylated microfibrillated cellulose fibers
with a particular level of freeness can achieve the aforementioned object. Therefore,
the aforementioned object is achieved by the following invention.
- (1) A carboxymethylated microfibrillated cellulose fiber having a Canada standard
freeness of less than 200 mL and an average fiber diameter of not less than 500 nm.
- (2) The carboxymethylated microfibrillated cellulose fiber as set forth in (1), having
an electrical conductivity of not more than 500 mS/m, as measured at pH 8 in the form
of a 1% by weight solids concentration water dispersion.
- (3) The carboxymethylated microfibrillated cellulose fiber as set forth in (1) or
(2), having a degree of substitution of from 0.01 to 0.50.
- (4) The carboxymethylated microfibrillated cellulose fiber as set forth in any of
(1) to (3), having a cellulose type-I crystallinity of not less than 50%.
- (5) A composition comprising the carboxymethylated microfibrillated cellulose fiber
as set forth in any of (1) to (4) and water.
- (6) The composition as set forth in (5), further comprising a raw material pulp.
- (7) The composition as set forth in (5) or (6), further comprising a binder.
- (8) The composition as set forth in any of (5) to (7), further comprising a white
pigment.
- (9) A dry solid formed by drying the composition as set forth in any of (5) to (8).
- (10) A method of preparing the carboxymethylated microfibrillated cellulose fiber
as set forth in any of (1) to (4), the method comprising the steps of:
- (A) carboxymethylating a pulp,
- (B) wet-grinding the pulp.
ADVANTAGEOUS EFFECTS OF INVENTION
[0006] According to the present invention, there can be provided a microfibrillated cellulose
fiber that provides a composition having an excellent water retention ability and
exerts an effect to enhance paper strength when added to a paper.
DESCRIPTION OF EMBODIMENTS
[0007] The present invention provides a carboxymethylated microfibrillated cellulose fiber
having a Canada standard freeness of less than 200 mL and an average fiber diameter
of not less than 500 nm. In this invention, ranges "from X to Y" include both endpoints
X and Y.
1. Carboxymethylated microfibrillated cellulose fiber
(1) Carboxymethylated microfibrillated cellulose fiber
[0008] Microfibrillated cellulose (hereinafter also referred to as "MFC") fibers refer to
fibers having an average fiber diameter (also referred to as "average fiber width")
of not less than 500 nm, which are obtained by fibrillating a cellulose-based raw
material such as pulp. Carboxymethylated microfibrillated cellulose (hereinafter also
referred to as "CM-modified MFC") fibers refer to a MFC obtained by fibrillating a
CM-modified cellulose-based raw material. As described later, the fibrillation is
preferably carried out by mechanical treatment. However, considering that a CM-modified
pulp is easy to ravel, it is considered that the CM-modified pulp is more or less
fibrillated since fibers are finely broken or loosened through a series of CM process
steps (
esp., dehydration and washing after carboxymethylation). Therefore, in this invention,
the CM-modified MFC also includes a CM-modified pulp not subjected to mechanical treatment.
In this invention, the average fiber diameter refers to a length-weighted average
fiber diameter, which can be determined by an image analysis-based fiber analyzer,
such as a fiber tester produced by ABB Japan K.K. or a fractionator produced by Valmet
K.K. For example, the MFC is obtained by relatively gently defibrating or beating
a cellulose-based raw material using a beater, disperser or the like. Therefore, the
MFC has a larger fiber diameter than cellulose nanofibers obtained by intensely defibrating
a cellulose-based raw material by a high pressure homogenizer or the like, and has
a structure in which the fiber surface is efficiently fluffed (externally fibrillated)
while the fiber itself is left not microfiberized (internally fibrillated).
[0009] As mentioned above, the CM-modified MFC of the present invention may be a CM-modified
pulp obtained by chemically modifying (carboxymethylating) a pulp, but is preferably
a mechanically-treated, CM-modified microfibrillated cellulose fiber (hereinafter
also referred to as "mechanically-treated, CM-modified MFC") obtained by subjecting
a CM-modified pulp to further mechanical treatment such as defibration. In other words,
the CM-modified MFC of this invention has electrically charged carboxymethyl groups
on the fiber surface regardless of being subjected to mechanical treatment, and thus,
as compared to unmodified pulp, shows an increased water retention value and varying
affinity for different chemicals. However, the mechanically-treated, CM-modified MFC
obtained by mechanically treating a CM-modified pulp has an increased specific surface
area and thus enables the effects of this invention to be produced at high levels.
Since the mechanically-treated, CM-modified MFC is obtained by relatively gently defibrating
or beating a CM-modified cellulose-based raw material such as CM-modified pulp, strong
hydrogen bonding present between fibers is weakened by chemical modification. Thus,
as compared to a MFC obtained simply by mechanical defibration or beating, the mechanically-treated,
CM-modified MFC is characterized in that the fibers are easier to ravel, less damaged,
and internally and externally fibrillated in a moderate manner. Further, a water dispersion
obtained by dispersing the CM-modified MFC of this invention in water has high hydrophilicity,
high water retention ability, and high viscosity.
[0010] As mentioned above, the MFC differs in degree of fibrillation from a cellulose-based
raw material. It is generally not easy to quantify a degree of fibrillation, but the
present inventors found that the degree of fibrillation of a MFC can be quantified
based on its Canada standard freeness, water retention value, and transparency.
[0011] The CM-modified MFC of the present invention has a carboxymethyl group, which is
an anionic group introduced thereto, and thus the physical properties of the CM-modified
MFC, including affinity for water, vary depending on the type of carboxymethyl groups,
i.e., whether the carboxymethyl groups are of H-type or of salt type. The properties of
the CM-modified MFC can vary since the type of carboxymethyl groups is adjusted as
appropriate depending on the intended use. Unless otherwise stated, the fiber properties
of the CM-modified MFC of this invention are evaluated based on the measurements obtained
for a CM-modified MFC which provides an alkaline water dispersion, or more specifically
a water dispersion with 1% by weight concentration at pH 8. Since the CM-modified
MFC, unlike common pulp, has anionic substituents introduced thereto, said MFC can
advantageously be used as an additive such as dispersant or coagulant, taking advantage
of its anionic characteristics.
<Canada standard freeness>
[0012] The Canada standard freeness of the CM-modified MFC of the present invention, as
measured so as to ensure that the aforementioned requirements are met, is less than
200 mL, preferably not more than 150 mL. The lower limit of the freeness is not limited
but is preferably higher than 0 mL. The Canada standard freeness of the CM-modified
MFC of this invention can be adjusted by adjusting the degrees of processing into
short fibers, nanosizing and fibrillation during treatment of a CM-modified pulp used
as a raw material. As a result of intensive studies, the present inventors found that
by fibrillating a CM-modified pulp to reduce Canada standard freeness, a novel material
different from common chemical pulp or cellulose nanofiber (also referred to as "CNF")
can be obtained. In other words, since the CM-modified MFC of this invention has a
Canada standard freeness of less than 200 mL, said CM-modified MFC is presumed to
have a form of fully fibrillated fibers. Further, since a CNF obtained by intensely
defibrating a fiber to micronize it to the single-nanometer level has a Canada standard
freeness of 0 mL, the CM-modified MFC of this invention differs from a CNF. The CM-modified
MFC having a Canada standard freeness of less than 200 mL has an excellent water retention
ability and can advantageously be used as a water retaining material. Further, when
the CM-modified MFC of this invention is used as a papermaking additive, fibrils formed
on the surface of the CM-modified MFC may contribute to an increase in the points
of bonding between cellulose fibers, leading to formation of a strong network between
CM-modified MFCs or between a CM-modified MFC and a pulp, whereby a paper with enhanced
barrier properties and paper strength can be produced.
<Water retention value>
[0013] The water retention value of the CM-modified MFC of the present invention is preferably
not less than 300%. When the water retention value is less than 300%, the effect of
this invention, which is enhancing the water retention ability of a composition comprising
the CM-modified MFC of this invention, may not be fully obtained. Water retention
value is determined according to JIS P-8228:2018.
<Transparency in the form of water dispersion>
[0014] The CM-modified MFC of the present invention is characterized by having low transparency
when made into the form of a water dispersion prepared using water as a dispersion
medium. In this invention, the transparency refers to the transmittance of light at
a wavelength of 660 nm through a 1% (w/v) solids concentration water dispersion of
a material of interest (e.g., CM-modified MFC). The specific method of determining
transparency is as described below.
[0015] A dispersion of the CM-modified MFC (solids concentration: 1% (w/v), dispersing medium:
water) is prepared and determined for transmittance of light at 660 nm using the UV-VIS
spectrophotometer UV-1800 (produced by Shimadzu Corporation) equipped with rectangular
cells with an optical path length of 10 mm.
[0016] In the present invention, the transparency of the CM-modified MFC is preferably not
more than 40%, more preferably not more than 30%, still more preferably not more than
20%, yet more preferably not more than 10%. In general, the transparency of a cellulose-based
material increases when the material is nanosized while it retains its crystallinity.
In contrast, the CM-modified MFC of this invention has a low level of transparency
because said MFC is not so much highly nanosized and retains its fiber structure.
When the CM-modified MFC having a transparency of not more than 40% is internally
incorporated into a paper, the CM-modified MFC retains its fiber structure in the
paper, thereby reducing the occurrence of a decline in paper thickness or paper density
and enabling enhancement of paper strength without deterioration of rigidity.
<Electrical conductivity>
[0017] The electrical conductivity of the CM-modified MFC of the present invention is preferably
not more than 500 mS/m, more preferably not more than 300 mS/m, still more preferably
not more than 200 mS/m, yet more preferably not more than 100 mS/m, most preferably
70 mS/m, as measured under the condition of pH 8 in the form of a 1.0% by weight water
dispersion. The lower limit of the electrical conductivity is preferably not less
than 5 mS/m, more preferably not less than 10 mS/m. The electrical conductivity of
the CM-modified MFC is higher than that of a CM-modified cellulose-based material
used as a raw material. An electrical conductivity exceeding the upper limit means
that the concentration of metal and inorganic salts dissolved in a water dispersion
of a CM-modified cellulose-based material is above a specified value. When the concentration
of metal and inorganic salts in said material is low, electrostatic repulsion can
easily occur between fibers, promoting efficient fibrillation.
[0018] Hereunder, a method of preparing a CM-modified MFC will be described.
1) Cellulose-based raw material
[0019] Examples of cellulose-based raw materials include, but are not particularly limited
to, cellulose-based raw materials derived from plants, animals (e.g., sea squirt),
algae, microorganisms (e.g., Acetobacter), and microorganism products. Examples of
cellulose-based raw materials derived from plants include wood, bamboo, hemp, jute,
kenaf, farm waste products, cloth, and pulps (e.g., softwood (nadelholz) unbleached
kraft pulp (NUKP), softwood bleached kraft pulp (NBKP), hardwood (laubholz) unbleached
kraft pulp (LUKP), hardwood bleached kraft pulp (LBKP), softwood unbleached sulfite
pulp (NUSP), softwood bleached sulfite pulp (NBSP), thermomechanical pulp (TMP), softwood
dissolving pulp, hardwood dissolving pulp, recycled pulp, waste paper). Also, a cellulose
powder obtained by grinding such a cellulose-based raw material as mentioned above
may be used. The cellulose raw material used in the present invention can be any or
a combination of the aforementioned materials, but is preferably a cellulose fiber
derived from a plant or microorganism, more preferably a cellulose fiber derived from
a plant, still more preferably a wood-based pulp, most preferably a hardwood pulp.
[0020] The average fiber diameter of a cellulose fiber is not particularly limited. Commonly
used softwood kraft pulps have an average fiber diameter of about from 30 to 60 µm,
and hardwood kraft pulps have an average fiber diameter of about from 10 to 30 µm.
Other pulps after a common purification procedure have an average fiber diameter of
about 50 µm. For example, in the case of using a raw material obtained through purification
of a several centimeter-sized material such as chip, it is preferable to subject the
raw material to mechanical treatment by a disintegrator such as refiner or beater
to adjust average fiber diameter to not more than about 50 µm, more preferably not
more than about 30 µm.
2) Carboxymethylation
[0021] Carboxymethylation refers to introducing carboxymethyl groups into a cellulose-based
raw material via ether bonds. The carboxymethyl groups may be introduced in the form
of a salt (-CH2-COOM, where M is a metal atom). Carboxymethylation is also referred
to as etherification. The following provides a detailed description of etherification.
<Cellulose type-I crystallinity>
[0022] With regard to the cellulose crystallinity of the CM-modified MFC of the present
invention, type-I crystals are preferably present at a concentration of not less than
50%, more preferably not less than 60%. By adjusting crystallinity within the aforementioned
range, the CM-modified MFC can exhibit effects, including imparting water retention
properties, when added to a paper. Further, when type-I crystals are present at a
concentration of not less than 50% in a CM-modified pulp as a raw material, the pulp
can be efficiently fibrillated by beating or defibrating treatment while it retains
its fiber structure, whereby the CM-modified MFC of this invention can be prepared
efficiently. Cellulose crystallinity can be controlled by the concentration of a mercerizing
agent, the temperature of mercerization treatment, and the degree of carboxymethylation.
Since a high concentration of alkali is used for mercerization and carboxymethylation,
cellulose type-I crystals are likely to convert to type-II. However, by controlling
the degree of modification through, for example, adjusting the amount of an alkali
(mercerizing agent) used, desired crystallinity can be maintained. The upper limit
of cellulose type-I crystallinity is not particularly limited. In practice, said upper
limit is presumed to be about 90%.
[0023] The method of determining the cellulose type-I crystallinity of a CM-modified MFC
is as described below.
[0024] A sample is placed into a glass cell and subjected to measurement using an X-ray
diffractometer (LabX XRD-6000, produced by Shimadzu Corporation). The calculation
of crystallinity is performed by a method such as Segal -- the crystallinity is calculated
according to the following equation based on the diffraction strength of plane (002)
at 2θ = 22.6° and the diffraction strength of amorphous region at 2θ = 18.5°, with
the diffraction strength at 2θ = 10° to 30° in an X-ray diffraction diagram being
used as a baseline.
Xc: Cellulose type-I crystallinity (%)
I002c: Diffraction strength of plane (002) at 2θ = 22.6°
Ia: Diffraction strength of amorphous region at 2θ = 18.5°.
[0025] CM-modified cellulose can generally be prepared by treating (mercerizing) a cellulose
material with alkali and then reacting the resulting mercerized cellulose (also referred
to as "alkali cellulose") with a carboxymethylating agent (also referred to as "etherifying
agent"). In the thus-obtained CM-modified cellulose, any of the hydroxyl groups located
at C2, C4 and C6 position in pyranose rings is carboxymethylated. In general, carboxymethylcellulose
(CMC), which is formed by dry-grinding a CM-modified cellulose, is characterized by
having water swellability, high safety and the like, and is used as an additive for
cosmetics and food products. Therefore, similarly to CMC, the CM-modified MFC of the
present invention, which is prepared using a CM-modified cellulose as a raw material,
can also be advantageously used as an additive for food products and cosmetics.
[0026] The degree of carboxymethyl substitution per anhydrous glucose unit in a CM-modified
cellulose or MFC obtained by carboxymethylation is preferably not less than 0.01,
more preferably not less than 0.05, still more preferably not less than 0.10. The
upper limit of this degree is preferably not more than 0.60, more preferably not more
than 0.50, still more preferably not more than 0.4. Therefore, the degree of carboxymethyl
substitution is in the range of preferably from 0.01 to 0.50, more preferably from
0.05 to 0.40, still more preferably from 0.10 to 0.30. In general, a CM-modified cellulose
has higher affinity for water and higher swellability in water as the degree of carboxymethyl
substitution becomes higher and the cellulose type-I crystallinity becomes lower.
However, the present inventors found that when a CM-modified pulp obtained by performing
a carboxymethylation reaction without deteriorating crystallinity is used as a raw
material and the CM-modified pulp is beaten or defibrated in a high water content
state, there can be obtained a CM-modified MFC which is fibrillated while retaining
its fiber structure.
[0027] The method of carboxymethylation is not particularly limited, and examples thereof
include such a method as mentioned above, in which a cellulose raw material used as
a starting material is subjected to mercerization followed by etherification. For
the carboxymethylation reaction, a solvent is generally used. Examples of a solvent
include water, alcohols (
e.g., lower alcohol) and mixed solvents thereof. Examples of a lower alcohol include methanol,
ethanol, N-propyl alcohol, isopropyl alcohol, N-butanol, isobutanol, and tertiary
butanol. As for the mixing proportion of a lower alcohol in a mixed solvent, the lower
limit is generally not less than 60% by weight, and the upper limit is not more than
95% by weight -- thus, said mixing proportion is preferably in the range of from 60
to 95% by weight. The amount of a solvent is generally 3 times by weight that of the
cellulose raw material. The upper limit of this amount is not particularly limited,
but is preferably 20 times by weight. Therefore, the amount of a solvent is preferably
in the range of from 3 to 20 times by weight.
[0028] Mercerization is generally performed by mixing a starting material with a mercerizing
agent. Examples of a mercerizing agent include alkali metal hydroxides such as sodium
hydroxide and potassium hydroxide. The amount of a mercerizing agent used is preferably
not less than 0.5 times moles, more preferably not less than 1.0 times mole, still
more preferably not less than 1.5 times moles, per anhydrous glucose residues in a
starting material. The upper limit of this amount is generally not more than 20 times
moles, preferably not more than 10 times moles, more preferably not more than 5 times
moles. Therefore, the amount of a mercerizing agent used is in the range of preferably
from 0.5 to 20 times moles, more preferably from 1.0 to 10 times moles, still more
preferably from 1.5 to 5 times mole.
[0029] The reaction temperature for mercerization is generally not less than 0°C, preferably
not less than 10°C, and the upper limit of this reaction temperature is generally
not more than 70°C, preferably not more than 60°C. Therefore, this reaction temperature
is generally in the range of from 0 to 70°C, preferably from 10 to 60°C. The reaction
time for mercerization is generally not less than 15 minutes, preferably not less
than 30 minutes. The upper limit of this reaction time is generally not more than
8 hours, preferably not more than 7 hours. Therefore, this reaction time is generally
in the range of from 15 minutes to 8 hours, preferably from 30 minutes to 7 hours.
[0030] The etherification reaction is generally performed by adding a carboxymethylating
agent to the reaction system after mercerization. Examples of a carboxymethylating
agent include sodium monochloroacetate. The amount of a carboxymethylating agent added
is generally preferably not less than 0.05 times moles, more preferably not less than
0.5 times moles, still more preferably not less than 0.8 times moles, per glucose
residues in a cellulose raw material. The upper limit of this amount is generally
not more than 10.0 times moles, preferably not more than 5 times moles, more preferably
not more than 3 times moles. Therefore, this amount is in the range of preferably
from 0.05 to 10.0 times moles, more preferably from 0.5 to 5 times moles, still more
preferably from 0.8 to 3 times moles. The reaction temperature for etherification
is generally not less than 30°C, preferably not less than 40°C, and the upper limit
of this reaction temperature is generally not more than 90°C, preferably not more
than 80°C. Therefore, this reaction temperature is generally in the range of from
30 to 90°C, preferably from 40 to 80°C. The reaction time for etherification is generally
not less than 30 minutes, preferably not less than 1 hour, and the upper limit of
this reaction time is generally not more than 10 hours, preferably not more than 4
hours. Therefore, this reaction time is generally in the range of from 30 minutes
to 10 hours, preferably from 1 to 4 hours. During the carboxymethylation reaction,
a reaction solution may be stirred depending on the need.
[0031] The degree of carboxymethyl substitution per glucose unit in a CM-modified cellulose
is determined according to, for example, the following method: 1) about 2.0 g (absolute
dry) of a CM-modified cellulose is precisely weighted out and placed into a 300 mL
stoppered conical flask; 2) 100 mL of a mixed solution of 1000 mL of methanol and
100 mL of premium grade concentrated nitric acid is added, and shaking is continued
for 3 hours to convert a carboxymethylcellulose salt (CM-modified cellulose) to a
H-type CM-modified cellulose; 3) 1.5-2.0 g of the H-type CM-modified cellulose (absolute
dry) is precisely weighted out and placed into a 300 mL stoppered conical flask; 4)
the H-type CM-modified cellulose is wetted with 15 mL of 80% methanol, 100 mL of 0.1
N NaOH is added, and shaking is continued at room temperature for 3 hours; 5) excess
NaOH is back titrated with 0.1 N H2SO4 using phenolphthalein as an indicator; 6) the
degree of carboxymethyl substitution (DS) is calculated according to the following
equation.
A: Amount (mL) of 1 N NaOH required for neutralization of 1 g of H-type CM-modified
cellulose
F: Factor for 0.1 N H2SO4
F': Factor for 0.1 N NaOH
3) Mechanical treatment
[0032] At this step, a CM-modified pulp is mechanically defibrated, beaten or disintegrated
to an average fiber diameter of not less than 500 nm. Mechanical defibration, beating
or disintegration is referred to as "mechanical treatment", and defibrating or beating
a CM-modified pulp dispersed in water is referred to as "wet-grinding". Mechanical
treatment may be performed once, or may be performed two or more times by repeating
the same procedure or combining different procedures. In the case of performing mechanical
treatment two or more times, different procedures may be performed at any given timing,
and the apparatus to be used may be the same or different. This step can be performed,
for example, by any of the following procedures.
the water dispersion of a CM-modified pulp is concentrated to high concentration (not
less than 20% by weight) by dehydration or the like, and then subjected to beating;
the water dispersion of a CM-modified pulp is diluted to reduce concentration (less
than 20% by weight, preferably not more than 10% by weight), and then subjected to
mechanical treatment such as beating or defibration;
the CM-modified pulp is subjected to drying, followed by mechanical defibration or
beating;
the CM-modified pulp is subjected to preliminary dry-grinding into short fibers, followed
by mechanical defibration or beating.
[0033] Since the present invention aims at promoting fibrillation of a fiber while preventing
it from being processed into short fibers, it is preferable to perform mechanical
treatment once, and more preferable to treat a low-concentrated water dispersion of
a CM-modified pulp using a refiner or a high-speed disintegrator.
[0034] The apparatus used for mechanical treatment is not particularly limited, and examples
thereof include different types of apparatus, such as high-speed rotating type, colloid
mill type, high pressure type, roll mill type, and ultrasonic type. Specific examples
thereof that can be used include some types of apparatus which perform mechanical
treatment by causing a metal or blade moving around the axis of rotation on pulp fibers,
and other types of apparatus which perform mechanical treatment by means of the friction
between pulp fibers, as exemplified by high-pressure or ultrahigh-pressure homogenizer,
refiner, beater, PFI mill, kneader, disperser, and high-speed disintegrator.
[0035] In the case of defibrating or beating a CM-modified pulp dispersed in water, the
lower limit of the solids concentration of the CM-modified pulp in the water dispersion
is generally preferably not less than 0.1% by weight, more preferably not less than
0.2% by weight, still more preferably not less than 0.3% by weight. At such a solids
concentration, the relative amount of a dispersion medium to the amount of the CM-modified
pulp becomes appropriate, leading to greater efficiency. The upper limit of this concentration
is generally preferably not more than 50% by weight.
[0036] At this step, a CM-modified MFC is obtained. The average fiber diameter of a CM-modified
MFC is not less than 500 nm, preferably not less than 1 µm, more preferably not less
than 10 µm, in terms of length-weighted average fiber diameter. The upper limit of
the average fiber diameter is preferably not more than 60 µm, more preferably not
more than 40 µm. The average fiber length of a CM-modified MFC is preferably not less
than 300 µm, more preferably not less than 400 µm, in terms of length-weighted average
fiber length. The upper limit of the average fiber length is preferably not more than
3000 µm, more preferably not more than 1500 µm, still more preferably not more than
1100 µm, most preferably not more than 900 µm. According to the present invention,
since a raw material pulp is carboxymethylated beforehand and subjected to mechanical
treatment by a procedure that minimizes cutting of fibers, the pulp fibers can be
fibrillated without being cut to an extremely short length. Further, since the affinity
of cellulose fibers for water is increased by carboxymethylation, the CM-modified
MFC can exhibit low freeness in spite of having a long fiber length.
[0037] Length-weighted average fiber diameter and length-weighted average fiber length can
be determined using an image analysis-based fiber analyzer, such as a fiber tester
produced by ABB Japan K.K. or a fractionator produced by Valmet K.K. The average aspect
ratio of a CM-modified MFC is preferably not less than 10, more preferably not less
than 30. The upper limit of the average aspect ratio is not particularly limited,
and is preferably not more than 1000, more preferably not more than 100, still more
preferably not more than 80. The average aspect ratio can be calculated according
to the following equation.
[0038] It is preferable that the degree of substitution per glucose unit in the CM-modified
MFC obtained at this step should be the same as that of a CM-modified pulp used as
a raw material.
2. Composition
[0039] The composition of the present invention comprises a CM-modified MFC and water. The
composition of this invention, which comprises a CM-modified MFC and water as mentioned
above, can be widely used for any applications that require water retention. The composition
of this invention can be used to serve as, for example, a thickener, a gellant, a
shape retainer, an emulsion stabilizer, or a dispersion stabilizer. To be specific,
the composition of this invention can be used in papermaking raw materials (additive,
raw material pulp), food products, cosmetics, pharmaceuticals, agricultural chemicals,
toiletries, sprays, paints, and the like. However, it is preferred that the composition
of this invention should be used, in a paper production process, as a paper raw material
(pulp slurry) for use at a papermaking step or as a pigment coating liquid or clear
coating liquid for use at a coating step. Thus, these applications are described below
for instance.
(1) Pulp slurry
[0040] A pulp slurry comprises not only a CM-modified MFC and water, but also a raw material
pulp. The raw material pulp refers to a pulp that serves as a main component of a
paper. The pulp raw material for a base paper used in the present invention is not
particularly limited, and examples thereof that can be used include: mechanical pulps
such as ground pulp (GP), thermomechanical pulp (TMP) and chemithermomechanical pulp
(CTMP); waste paper pulps such as deinked pulp (DIP) and undeinked pulp; and chemical
pulps such as nadelholz (softwood) kraft pulp (NKP) and laubholz (hardwood) kraft
pulp (LKP). As waste paper pulps, use can be made of those pulps derived from sorted
waste papers such as high-quality paper, medium-quality paper, low-quality paper,
newspaper waste paper, leaflet waste paper, magazine waste paper, corrugated paper,
and printed waste paper, or those pulps derived from unsorted waste papers comprising
a mixture of different waste papers.
[0041] The content of a CM-modified MFC in a pulp slurry is preferably 1×10
-4 to 20% by weight, more preferably 1×10
-3 to 5% by weight, based on the amount of a raw material pulp. If this content exceeds
its upper limit, the water retention ability of the pulp slurry will become too high,
possibly causing poor water drainage at a papermaking step. If this content falls
below its lower limit, enhancement of water retention ability or enhancement of the
paper strength of a produced paper may not be achieved due to too small an amount
of a CM-modified MFC added.
[0042] The pulp slurry may contain a known filler. Examples of fillers include: inorganic
fillers such as heavy calcium carbonate, light calcium carbonate, clay, silica, light
calcium carbonate-silica composite, kaolin, fired kaolin, delaminated kaolin, magnesium
carbonate, barium carbonate, barium sulfate, aluminum hydroxide, calcium hydroxide,
magnesium hydroxide, zinc hydroxide, zinc oxide, titanium oxide, and amorphous silica
produced by neutralizing sodium silicate with a mineral acid; and organic fillers
such as urea-formalin resin, melamine resin, polystyrene resin and phenol resin. Such
fillers may be used alone or in combination. Among them, preferred is heavy calcium
carbonate or light calcium carbonate, which are representative fillers used to make
neutral and alkaline papers and can give papers high opacity. The content of a filler
is in the range of preferably from 5 to 20% by weight based on the amount of a raw
material pulp. In the present invention, it is more preferred that the content of
a filler should be not less than 10% by weight, since the decline in paper strength
can be reduced even when paper ash content is high.
[0043] The CM-modified MFC of the present invention can function as a paper strengthening
agent, a water retainer, or a yield improver in a pulp slurry. In addition to the
MFC of this invention, various wet end additives, including bulking agent, dry paper
strengthening agent, wet paper strengthening agent, freeness improver, dye, or cationic,
nonionic or anionic sizing agent, may be added to a pulp slurry depending on the need.
[0044] The pulp slurry of the present invention is prepared by any given method, but it
is preferable to add the CM-modified MFC at the step of subjecting a raw material
pulp to refining or mixing treatment. When the CM-modified MFC is added at a mixing
step, a mixture prepared beforehand by mixing the CM-modified MFC with a filler and
other auxiliary agents such as yield improver may be added to a raw material pulp
slurry.
[0045] The solids concentration of a pulp slurry is adjusted as appropriate depending on
papermaking conditions and the like, but is preferably in the range of from 0.1 to
1.0% by weight. Such a pulp slurry is made into a paper by a known papermaking method.
Papermaking can be carried out using, for example, but not limited to, a fourdrinier
paper machine, a gap former-type paper machine, a hybrid former-type paper machine,
an on-top former-type paper machine, or a cylinder paper machine.
(2) Clear coating liquid
[0046] The clear coating liquid is a coating liquid composed mainly of a water-soluble polymer
commonly used as a surface treating agent, including starch (
e.g., oxidized starch, modified starch, dextrin), carboxymethylcellulose, polyacrylamide,
or polyvinyl alcohol. In addition to the water-soluble polymer, various additives
such as water resisting agent, external sizing agent, surface strengthening agent,
dye or pigment, fluorescent colorant, and water retainer may be contained in a clear
coating liquid. The water-soluble polymer can also serve as a binder.
[0047] The content of a CM-modified MFC in a clear coating liquid is not particularly limited.
Total solids content may consist of a CM-modified MFC, but from viewpoints of coating
suitability and the like, it is preferred to use a CM-modified MFC in admixture with
the aforementioned water-soluble polymer. The mixing ratio of water-soluble polymer
and CM-modified MFC is in the range of preferably from 1:10000 to 10000:1, more preferably
about from about 1:1 to 500:1.
[0048] By coating one or both sides of a base paper with a clear coating liquid by a known
method, a clear coating layer can be formed. In the present invention, the term "clear
coating" refers to coating or impregnating a base paper with a clear coating liquid
using a coater such as size press, gate roll coater, premetered size press, curtain
coater, or spray coater. The coating amount of a clear coating layer is in the range
of preferably from 0.1 to 1.0 g/m
2, more preferably from 0.2 to 0.8 g/m
2, in terms of solids per one side.
(3) Pigment coating liquid
[0049] The pigment coating liquid is a composition comprising a white pigment as a main
component. Examples of a white pigment include commonly used pigments such as calcium
carbonate, kaolin, clay, fired kaolin, amorphous silica, zinc oxide, aluminum oxide,
satin white, aluminum silicate, magnesium silicate, magnesium carbonate, titanium
oxide, and plastic pigments.
[0050] The content of a CM-modified MFC in a pigment coating liquid is preferably in the
range of 1×10
-3 to 1 part by weight based on 100 parts by weight of a white pigment. When this content
falls within the aforementioned range, there can be obtained a pigment coating liquid
having moderate water retention ability without showing a significant increase in
viscosity.
[0051] The pigment coating liquid contains a binder. Examples of a binder include, but are
not limited to: different types of starches, such as oxidized starch, cationic starch,
ureaphosphoric acid esterified starch, etherified starch (e.g., hydroxyethyl etherified
starch), and dextrin; different types of proteins, such as casein, soybean protein,
and synthetic protein; polyvinyl alcohol; cellulose derivatives such as carboxymethylcellulose
and methylcellulose; conjugated diene polymer latexes, such as styrene-butadiene copolymer
and methyl methacrylate-butadiene copolymer; acrylic polymer latexes; and vinyl polymer
latexes such as ethylene-vinyl acetate copolymer. Such binders may be used alone,
or two or more thereof may be used in combination. It is preferable to use a starch-based
binder and a styrene-butadiene copolymer in combination.
[0052] The pigment coating liquid may contain different auxiliary agents commonly used in
the field of paper production, such as dispersant, thickener, antifoamer, colorant,
antistatic agent, or antiseptic agent.
[0053] By coating one or both sides of a base paper with a pigment coating liquid by a known
method, a pigment coating layer can be formed. From the viewpoint of coating suitability,
the solids concentration of a pigment coating liquid is preferably in the range of
about from 30 to 70% by weight. One, two or three or more pigment coating layers may
be formed. When there are two or more pigment coating layers, it is only necessary
that any one of the layers should be formed with a pigment coating liquid comprising
a CM-modified MFC. The coating amount of a pigment coating layer is adjusted as appropriate
depending on the intended use, but in the case of production of a coated paper for
printing, said coating amount is not less than 5 g/m
2, preferably not less than 10 g/m
2, per one side in total. The upper limit of this coating amount is preferably not
more than 30 g/m
2, more preferably not more than 25 g/m
2.
(3) Dry solid
[0054] The composition of the present invention can be dried into a dry solid. In particular,
a dry solid (base paper, clear coating layer, pigment coating layer) obtained by drying
a water dispersion comprising a raw material pulp, a water-soluble polymer, a white
pigment or the CM-modified MFC of this invention has both strength and pliableness.
The reason for this is not known but is presumed to be as follows. Since a water dispersion
of the CM-modified MFC of this invention is defibrated in a gentler manner than a
CNF which is defibrated to the single-nanometer level, the CM-modified MFC of this
invention is dispersed in water while it has a fibrillated surface but retains its
fiber structure. Therefore, a dry solid obtained by drying such a water dispersion
contains a fiber network which is reinforced by hydrogen bonds formed between fibrillated
fibers, and thus combines strength and pliableness. Said dry solid can be used as
a composition when water is added thereto.
3. Paper comprising a CM-modified MFC
[0055] Since a pulp slurry comprising the CM-modified MFC of the present invention has high
water retention ability, a paper made from such a pulp slurry has high paper strength
and high air resistance. Also, a paper having a pigment coating layer or clear coating
layer formed from a pigment coating liquid or clear coating liquid comprising the
CM-modified MFC of this invention shows a reduced degree of penetration of a binder
into a base paper, and thus has high coating layer strength and high air resistance.
[0056] A paper comprising the CM-modified MFC of the present invention preferably has a
base weight of from 10 to 400 g/m
2, more preferably from 15 to 100 g/m
2. A base paper used to produce a paper comprising the CM-modified MFC of this invention
may be composed of a single layer or of multiple layers. A paper made from a pulp
slurry comprising the CM-modified MFC has a base paper layer comprising the CM-modified
MFC. When the paper has multiple base paper layers, it is only necessary that at least
any one of these layers should comprise the CM-modified MFC. Further, the ash content
of said paper varies with the presence or absence of a pigment coating layer, but
this ash content is preferably in the range of from 0 to 30% for a paper having no
pigment coating layer (
i.e., base paper or clear coated paper), and in the range of from 10 to 50% for a paper
having a pigment coating layer.
[0057] A paper comprising the CM-modified MFC may have a clear coating layer depending on
the need. Also, a paper comprising the CM-modified MFC may be subjected to surface
treatment or other treatments by a known method.
EXAMPLES
[0058] Hereunder, the present invention will be described by way of examples. Analysis of
physical properties was performed according to the following procedures.
[0059] Average fiber length, average fiber diameter: A 0.2% by weight slurry was prepared
by adding ion exchange water to a sample and determined for these properties using
a fractionator produced by Valmet K.K.
[0060] Canada standard freeness (c.s.f.): This property was determined according to JIS
P 8121-2:2012.
[0061] Electrical conductivity: A water dispersion with a sample (e.g., CM-modified MFC)
concentration of 1.0% by weight was prepared and determined for electrical conductivity
at pH 8 using a portable electrical conductivity meter produced by Horiba Ltd.
[0062] Base weight: This property was determined according to JIS P 8223:2006.
[0063] Bulk thickness and bulk density: These properties were determined according to JIS
P 8223:2006.
[0064] Specific burst index: This property was determined according to JIS P 8131:2009.
[0065] Specific tensile strength: This property was determined according to JIS P 8223:2006.
[0066] Tensile elongation at break and specific tensile energy absorption: These properties
were determined according to JIS P 8223:2006 and JIS P 8113:1998.
[0067] Short-span specific tensile strength: This property was determined according to JIS
P 8156:2012.
[0068] Air resistance: This properties was determined according to JIS P 8117:2009 using
an Oken air resistance-smoothness tester.
[Examples A1, A2] Preparation of CM-modified MFCs
[0069] A stirrer capable of mixing pulp was charged with 200 g by dry weight of a pulp (NBKP
(softwood bleached kraft pulp), produced by Nippon Paper Industries Co., Ltd.) and
111 g by dry weight of sodium hydroxide, and water was added to give a pulp solids
content of 20% by weight. Thereafter, the mixture was stirred at 30°C for 30 minutes,
and then 216 g of sodium monochloroacetate (in terms of active component content)
was added thereto. The resulting mixture was stirred for 30 minutes, heated to 70°C,
and further stirred for 1 hour. Thereafter, the reaction product was taken out, neutralized
and washed to obtain a CM-modified pulp of Example A1 having a degree of carboxymethyl
substitution per glucose unit of 0.25.
[0070] The obtained CM-modified pulp was dispersed in water to form a 4% by weight water
dispersion, which was treated in a high-speed disintegrator (product name: TopFiner,
produced by Aikawa Iron Works Co., Ltd.) to obtain a CM-modified MFC of Example A2.
The physical properties of the obtained CM-modified MFC are shown in Table 1.
[Comparative Examples A1, A2]
[0071] A NBKP pulp treated by the same procedure as in Example A2 was obtained, except that
a non-CM-modified pulp (NBKP, produced by Nippon Paper Industries Co., Ltd.) was used
and the high-speed disintegrator was replaced with a single-disc refiner (product
name: 14 Inch Labo Refiner, produced by Aikawa Iron Works Co., Ltd.). The physical
properties of the treated pulp and the NBKP used as a raw material are shown in Table
1. In this table, the NBKP used as a raw material is denoted as Comparative Example
A1.
[Example B1]
[0072] 96% by weight of a corrugated waste paper (produced by Nippon Paper Industries Co.,
Ltd.) and 4% by weight of the CM-modified MFC (≤ 10 mL c.s.f.) prepared in Example
A2 were mixed to give a mixed pulp with a solids concentration of 0.8% by weight.
Based on the total amount of the mixed pulp, 1.0% by weight of aluminum sulfate, 0.15%
by weight of polyacrylamide, and 0.2% by weight of a sizing agent were added to prepare
a stock. The prepared pulp slurry was used to make a handmade sheet with an aim to
achieve a base weight of 100 g/m
2, and the handmade sheet was subjected to analysis. The handmade sheet was made according
to JIS P 8222.
[Comparative Examples B1, B2]
[0073] Handmade sheets were made and analyzed by the same procedure as in Example B1, except
that no CM-modified MFC was used. The corrugated waste paper used in Comparative Example
B1 was of the same lot as that used in Example B1, and the corrugated waste paper
used in Comparative Example B2 was of the same lot as that used in Comparative Example
B3.
[Comparative Example B3]
[0074] A handmade sheet was made and analyzed by the same procedure as in Example B1, except
that the pulp obtained in Comparative Example A2 was used instead of a CM-modified
MFC. The physical properties of this handmade sheet are shown in Table 2.
[Table 1]
|
Ex. A1 |
Ex. A2 |
Com Ex. A1 |
Com Ex. A2 |
Type |
CM-modified MFC |
Untreated NBKP |
Treated NBKP |
Raw material |
CM-modified pulp (Na) |
NBKP |
Average fiber length |
mm |
0.77 |
0.58 |
1.72 |
1.62 |
Average fiber diameter |
µm |
15.2 |
14.2 |
16.6 |
17.6 |
CSF |
ml |
52 |
≤10 |
620 |
156 |
Electrical conductivity |
mS/m |
28 |
53 |
5 |
13 |
[Table 2]
|
Com. Ex. B1 |
Ex. B1 |
Com. Ex. B2 |
Com. Ex. B3 |
Raw material |
Type |
Corrugated waste paper |
Amount added |
wt.% |
100 |
96 |
100 |
96 |
Refiner-treated pulp |
|
- |
Ex. A2 |
- |
Com. Ex. A2 |
Amount added |
wt.% |
0 |
4 |
0 |
4 |
Bulk thickness |
mm |
0.161 |
0.151 |
0.163 |
0.159 |
Bulk density |
g/cm3 |
0.63 |
0.67 |
0.62 |
0.64 |
Specific burst index |
kPa·m2/g |
3.27 |
3.82 |
3.11 |
3.13 |
Specific tensile strength |
N·m/g |
39.6 |
51.3 |
39.6 |
41.7 |
Tensile elongation at break |
% |
2.2 |
2.6 |
2.1 |
2.1 |
Specific tensile energy absorption |
J/kg |
629 |
956 |
577 |
612 |
Short-span specific tensile strength |
kN·m/kg |
23.9 |
27.6 |
23.7 |
24.8 |
Air resistance (Oken) |
sec |
25 |
82 |
25 |
31 |
[0075] It is apparent that the paper of the present invention has excellent paper strength
and air resistance.